1. Field of the Invention
The present invention relates to a vacuum generator for leading an intake negative pressure of a combustion engine to a negative pressure device and for increasing the vacuum pressure in the negative pressure device with another negative pressure which is generated at a vacuum takeout port of an ejector through which a part of the intake air to the engine flows.
2. Discussion of the Related Art
Generally, in a vacuum generator incorporated in a combustion engine of this type, the downstream of a throttle valve arranged in a suction pipe of the combustion engine is connected through a check valve to a negative pressure chamber of a vacuum booster incorporated in a brake booster device. An ejector is connected to the suction pipe to bypass the throttle valve, and a vacuum takeout port of the ejector is connected to the negative pressure chamber of the vacuum booster through another or second check valve. In the vacuum generator of this construction, the air volume which flows through the ejector to bypass the throttle valve varies in dependence on the ambient temperature and the atmospheric pressure. This causes an actual intake air volume to vary with respect to the opening degree of the throttle valve which is to attain a desired intake air volume of the combustion engine, so that a problem arises in that the control of the combustion engine becomes unstable.
Japanese unexamined, published patent application No. 2002-371885 describes a vacuum generator designed to solve this problem. In the known device, the air volume which flows through an ejector is calculated based on the area for a flow passage of the ejector, the ambient temperature and the atmospheric pressure. The opening degree of a throttle valve is compensated in dependence on the air volume which bypasses the throttle valve, so that the intake air volume to a combustion engine can be controlled to a desired volume.
However, in the aforementioned vacuum generator, a temperature sensor and an atmospheric pressure sensor have to be provided for measuring the ambient temperature and the atmospheric pressure, and the air volume that flows through the ejector has to be obtained by calculation. This gives rise to another problem that the vacuum generator becomes complicated in construction and increased in cost.